There has been an increasing demand for color filters mainly for liquid crystal displays mounted on laptop computers. In addition, demand for color filters for car navigation systems, video cameras, or mobile computing terminals has also been increasing recently. Since these devices are used outdoors in most cases, improved heat resistance and heat stability of the color filter are required, compared to those for computers mainly used indoors.
As a base material for a colored coating film in a color filter, acrylic resins or polyimide reins are usually employed. Polyimide resins have excellent heat resistance, but have poor properties in pattern development. On the other hand, acrylic resins has excellent properties in pattern development, but have inferior heat resistance to that of polyimide, which will cause discoloration or weight loss of the coating films in pixels, when the substrate is left under high temperature and humidity for a prolonged period of time.
In order to improve the heat resistance of an acrylic resin, efforts are made usually to raise the glass-transition temperature of the resin itself by a variety of means, for example, by increasing the molecular weight of the acrylic resin, or by increasing the content of methacrylic monomers. However, these techniques have disadvantages in that the viscosity of the resin is considerably increased, which leads to poor storage stability of the colored coating material and insufficient handling properties in preparation of the coating material.
Among the acrylic resins, known as a composition having excellent weatherability and heat resistance is an electrodeposition coating material composition used for vehicles, aluminum material for building, domestic electric appliances, or office machines to improve anti-corrosion properties and surface finish, disclosed, for example in Japanese Laid-open Patent Application Nos. 1-144473 and 1-304162. These publications disclose, in particular, that resins obtained by mixing and polymerizing particular amounts of a (meth)acrylic monomer having an acid group, a (meth)acrylic monomer having a hydroxyl group, a (meth)acrylate monomer, and N-alkyl substituted maleimide exhibit excellent resistance to heat and chemicals, as well as superior adhesivity to a material to be coated. However, the electrodeposition coating materials disclosed in these publications are merely clear varnish without any pigments contained. In addition, the thickness of the coating film to be formed is as thick as 20 to 35 .mu.m. Therefore, it is not intended to use these reins for a color filter. Still less, the properties of the resins when mixed with coloring pigments have not been studied.